GRADING AND SCREENING
the score of good screening the length of the path and within certain limits the slowness with which this path is traversed are the important considerations. It has already been pointed out that the more apertures the undersize grains pass over the better the chance of their elimination and again that in order to afford them the opportunity to fall through the apertures they must not proceed over the screen at too great rate of speed. It must be evident that the saw-toothed path pursued by any grain in the bank is of great length and that the grains pass over such a path very slowly. It is only necessary to exercise the imagination slightly on the great length of the saw-tooth path if it is pulled out into a straight line to appreciate this argument. On the other hand owing to the heaping up of the grains in a bank many of the grains have little or no opportunity to come in contact with the screen surface or on reaching the screen surface remain there only momentarily or but a short time. Although the path of any particle in the bank is long its path in contact with the screen may be short and haphazard or it may not reach the screen at all. No theory or mathe-
matieal analysis can properly weigh the two opposing factors of capacity in the tonnage sense and good screen work, experience must be the guide.
AH a capacity screen the trommel will do as good work as a flat screen of equal width. Thus a revolving screen of 4 ft. diameter would have the capacity of a flat •screen 4 ft. wide. The trommel as a capacity screen must be made longer than the flat, screen but- the great lengths of trommel which were formerly used are no longer in favor. As the length has boon reduced the slope has been increased as it has been found t his flattens the bunk and gives better screening effect. As with the flat screen, for capacity screening extra length has been found to add so little extra screening effect that it, does not outweigh the other disadvantages attending it. A 6-ft. length is very popular today.
Centrifugal Reels. -For light, non-abrasive, finely divided dry stock with little oversize centrifugal reels are to be recommended. These are placed with their supporting shaft horizontally or vertically and run at such a speed as to cause the material fed to them to occupy the whole surface of the screen owing
1 Courtesy Groat Western MfjK. Co.